考虑流体介质影响的管路模态特性分析

针对航空发动机外部管路的动力学设计需求及湿模态特性求解问题。应用铁摩辛柯梁理论和能量法推导得到流体质量、压强、流速等综合影响下的管路湿模态振动方程，并基于软件ANSYS现有管单元，通过引入自定义附加刚度和阻尼单元建立了流体压强和流速影响项的等效方法，实现了考虑流体因素影响的管路模态特性计算。结果表明:流体压强引起的横向压力差会降低管路弯曲刚度，而压强引起的轴向张力会提高弯曲刚度，压强作用效果与管支点的轴向约束程度和管材泊松比有关。流体速度对管路湿模态影响与边界条件、振型和管路轴向伸长有关，在高流速时，模态频率可能为零，即出现屈曲失稳现象。 

Modal characteristics analysis for pipelines considering influence of fluid medium

According to the requirement for the dynamic design of the external pipelines of aero-engine, and for solving the wet modal characteristics of complex pipelines, the Timoshenko beam theory and the energy method were used to derive the equation of the vibration of pipeline, in consideration of the effects of mass, pressure and velocity of fluid. Based on the existing pipe element in the ANSYS, an equivalent method to consider the pressure and velocity of fluid was established by using the stiffness and damping element, and it can be used in wet modal calculation of pipelines in ANSYS. Results showed that, the lateral force difference caused by the pressure of fluid could reduce the bending stiffness of the pipeline, and the axial tension caused by pressure could increase the bending stiffness; the effect of the pressure was related to the degree of axial constraint of the supports, and the Poissons ratio. The effect of the fluid velocity on the wet modal of the pipeline was related to the boundary conditions, the modes of vibration and the axial elongation of the pipeline. At high velocity, the modal frequency was zero, that was, the buckling instability occurring.